Use of generalized linear models to analyze catch rates having zero values: the Kuwait driftnet fishery

Abstract

Fishery survey data often contain zero values. This paper uses a gamma-based generalized linear model for non-zero values and a Bernoulli-based model for the proportion of non-zero catches. Yearly variation in catch rates is then estimated by integrating the estimates from both models. The results for the Kuwait driftnet fishery show that the catch rate of silver pomfret ( Pampus argenteus) decreased from 115 kg per 1000 m clearances in 1984 to about 10 kg per 1000 m clearances in 1999, but its alternative target species, hilsa shad ( Tenualosa ilisha), exhibited no clear pattern with great fluctuation over time. The proportion of non-zero catches for silver pomfret decreased with time during the study period. In contrast, the proportion for hilsa shad increased, indicating fishermen shifted their targeting from silver pomfret to hilsa shad with the decreasing catch rate of silver pomfret. Other effect factors like month, boat type and landing port were also included in the models, and their effects on catch rates were estimated and tested.